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Tombstoning effect (also known as Manhattan effect, Drawbridge effect, or Stonehenge effect) is considered a common soldering defect in surface mount electronic assembly of small leadless components such as resistors and capacitors. Recently, the trend toward miniaturization in electronic assembly to achieve smaller, lighter, and higher performing products has resulted in rapidly increasing implementation of small leadless passive and active components. Until recently, the 0603 components (this terminology means that the components are 6 milxc3x973 mil in size), which have been prevalent for years in high volume production, have produced very high yield and few defects. The 0402 and 0201 components have recently been used more frequently and have presented electronic assemblers the tremendous challenge of decreasing the defects due to increasing use of these components in assembly.
The tombstoning effect is due to the imbalance of the surface tension of the molten solder at both ends of the component during reflow soldering. Because of the small dimensions of these 0402 and 0201 components, the intricate balance of the surface tension may be more easily disturbed by either the change of the solderability of the components or by the differences of time at which the solder paste at each end of the component begins to melt.
One approach to solving the tombstoning problem has been proposed by Taguchi et al (U.S. Pat. No. 6,050,480), which teaches using a solder powder comprising of a solder alloy consisting of 60-65% Sn, 0.1-0.6% Ag, 0.1-2%Sb, and a balance of Pb, to prevent tombstoning during reflow soldering. The essence of Taguchi is to employ Ag and Sb to effectively increase the solidification temperature range and, in turn, to prevent the tombstoning.
The object of this invention is to employ a solder alloy comprising tin/lead/silver in order to provide a wider solidification range and achieve balance between the surface tension of both side of a small leadless component. The expanded solidification range slows the melting and wetting time so as to balance the surface tension of the molten solder, and in turn reduces the tombstoning frequency. The preferred Ag concentration is 0.2-0.5% in weight, while the more preferred Ag concentration is 0.3-0.4% in weight. For reflow soldering of small leadless components, the pastes made with the alloy compositions of Sn62.6Pb37Ag0.4 and Sn63Pb36.6Ag0.4 result in minimization of tombstoning.
It has been discovered that the addition of a small amount of silver to a tin/lead solder composition can dramatically reduce the tombstoning, particularly with the extremely small electronic components that have recently been used in electronic assemblies. In particular, it has been found that the amount of silver should be in the range of 0.1-0.7%, preferably 0.2-0.5% (for a tin/lead alloy in the range of approximately Sn63Pb37), more preferably 0.3-0.4%.
This unexpected benefit has been found in solders where the tin content ranges from about 58.0-68.0% (preferably 61.0-65.0% and most preferably 62.0-64.0%), while the lead content can range from about 32.0-42.0% (preferably 35.0-39.0% and most preferably 36.0-38.0%). (Note: it is recognized that, in these three-component tin/lead/silver solders, the maximum amount of each of the three components cannot be used, since the total must not exceed 100%.)
Thus, preferred alloys have been found to be the compositions
a. Sn62.6Pb37Ag0.4,
b. Sn63Pb36.6Ag0.4,
c. Sn62Pb37.6Ag0.4,
d. Sn62.2Pb37.4Ag0.4
In addition, it has been found that deviations from the desired silver content of 0.3-0.4% has resulted in increased tombstoning, as shown in the following table 1. In this test a rosin mildly activated flux was used, the metal load was 90%, and the powder size of the Sn63Pb37 powder was 45-25 microns (i.e., particles passed through 325 mesh, but failed to pass through 500 mesh screens). (Note: this test employs somewhat exaggerated conditions. Under these test conditions, the acceptable range of silver is from 0.1-0.5%. Under normal (i.e., non-test) conditions, we have found acceptable performance using 0.1-0.7% silver.)
In addition it has been found that Sb does not help reduce the tombstoning frequency. Taguchi teaches using amounts of Ag (0.1-0.6%) and Sb (0.1-2%) to reduce the tombstoning frequency. Note that Taguchi indicates that the presence of less than 0.1% of either of these metals will result in the loss of the antitombstoning benefit. However, in the instant invention, it has been found that the Sb is not needed to reduce tombstoning. (Note that, in analyzing the alloys in the instant invention, only a slight contaminant level of Sb (less than 0.01%) was found.)
Further investigations of the effect of elements such as Ag, Bi, In, Sb, Zn, Cu, and Ge on the expansion of the solidification range of the Sn63Pb37 are tabulated as follows:
It is very clear from table 2 that the addition of 0.4% of Ag resulted in the most expansion of the solidification range (the upper end minus the lower end of the melting range), while the addition of Sb resulted in a much smaller solidification range.
Accordingly, there is little benefit of employing Sb in antitombstoning applications.
Another benefit resulting from the exclusion of antimony is the elimination of exposure to this hazardous material. (For information on the toxicity of Sb and the gastrointestinal and respiratory effect of Sb on humans, see Antimony and Compounds, United States Environmental Protection Agency, Office of Air Quality Planning and Standards. Unified Air Toxics Website. www.epa.gov/ttn/uatw/hlthef/antimony.html.)
Furthermore, the inclusion of one fewer component certainly results in simplification and reduction in cost of the manufacturing and quality control processes. In addition, it is well known that use of more complex alloys (greater than 3 elements) generally results in significant variations of alloy compositions of solder joints. (Chris Bastecki, xe2x80x9cA Benchmark Process for the Lead-free Assembly of Mixed Technology PCB""s xe2x80x9d Revised 1999, Alpha Metals website, www.alphametals.com)
In a more severe case, a larger pasty range (resulting from an alloy with more than 3 components) could result in xe2x80x9chot tearingxe2x80x9d as the solder joints experience thermal excursions. Furthermore, it was also observed that in order to avoid the formation and segregation of low melting point phases, which could cause cracking or hot tearing, the alloy composition should be as close to a eutectic as possible. (Tommi Laine-Ylijorki, Atso Forsten, and Dr. Hector Steen, xe2x80x9cDevelopment and Validation of a Lead-free Alloy for Solder Paste Applicationsxe2x80x9d, Future Circuits International, Vol. 2, issue 1, p. 183-185.)
In addition, the solder alloys needed to be close to the eutectic composition because of smaller melting range, lower viscosity, and superior mechanical properties compared with off-eutectic compositions. (J. H. Vincent and G. Humpston, xe2x80x9clead-free Solders for Electronic Assemblyxe2x80x9d, GEC Journal of Research, Vol. 11, No. 2, 1994, page 76.)
Multi-component doping elements added to the eutectic Sn63Pb37 composition caused greater deviation from the eutectic composition than one component, and thus suffered the above-cited weaknesses.
In the past, 0.2-0.5% Sb had been added to solders to reduce the tin pest problem, which is the allotropic transformation of beta-tin into alpha-tin at temperatures below 13xc2x0 C. However, at least in Sn63Pb37 alloys, we have not observed the tin pest problem. (American National Standard, ANSI/J-STD-006 January 1995, p. 14.).
Thus, it is preferred to eliminate Sb from the alloy in order to improve performance of the solder alloy.
It is recognized that formulations similar to those disclosed herein have been described, but they are used for significantly different purposes. Rohde and Swearengen (Deformation Modeling Applied to Stress Relaxation of Four Solder Alloysxe2x80x9d, J. Eng. Materials and Technology (ASME) 102, April 1980 p. 207-214)) examined four alloys Pb50In50, Sn37.5Pb37.5In25, Sn63Pb37, and Sn62.5Pb37Ag0.5 for the purpose of obtaining parameters for establishing a stress relaxation model to describe the thermal fatigue phenomenon of solder joints between electrical component leads and printed circuit board. In contrast, anti-tombstoning alloys deal with solder joints between small leadless components and printed circuit board. Although the Sn62.5Pb37Ag0.5 is within the range of the present invention, the intended purpose of the Rohde and Swearengen""s work is completely different from the present invention.
It is preferred to control the concentration of Ag to within 0.1 percent of the preferred 0.3-0.4 percent. Results have shown that at only 0.2 percent deviation from the preferred 0.4 percent Ag, the frequency of tombstoning increased at least 30%, as shown in Table 1.
The benefit of reducing tombstoning by adding silver to other tin/lead solders aside from the alloys discussed above has also been observed. However, these alloys are less frequently used for surface mount leadless components and, as a result, tombstoning is generally less of a problem for these alloys.
The following examples are intended to illustrate, but not limit, the invention.